The term xylem (Greek, xylos = wood) was introduced byNageli (1858). Chief conducting tissue of vascular plants responsible for conduction of water and inorganic solutes.
Components of Xylem
(i) Tracheids :
Tracheids are elongated tube like dead cells (without protoplasm) with tapering ends (tangenital section)
They appear elongated (tangenital section) rectangular or somewhat rounded (radial section), angular or polygonal (cross section) but in some cases they may appear rounded. The walls are hard and lignified but not much thick and enclose a wide empty lumen. The tracheids are long but not as long as fibres. They reach up to 1 mm in length (in some plants they may attain a length of 12 cm or more).
The tracheids of primary xylem developes from pro-cambium whereas those of secondary xylem developes from vascular cambium. In the beginning these cells possess living protoplasm but due to lignification and deposition of thickening materials in wall they become dead at maturity.
Wall thickenings in Tracheids :
Tracheids possess various types of wall thickenings viz – annular, spiral, scalariform, reticulate and pitted. The protoxylem tracheids have annular (ring-.like) and spiral (helical) thickenings. The metaxylem and secondary xylem tracheids have scalariform (ladder like), reticulate (network) and pitted thickenings. The pits are of two types simple or bordered. The size and number of pits vary greatly in each tracheid.
Functions of Tracheids:
- To conduct water and dissolved mineral elements from roots to the leaves. They are structurally adapted to their specific function.They are placed one above the other and also parallel to the long axis. The end walls are perforated by the presence of bordered pits which permit flow of water from one cell to another.
- Also provide mechanical support due to presence of hard and firm secondary walls.
